The 26-year-old is seeking regular football, having played just three times for the Seagulls this year.
Brighton manager Gus Poyet told the club's website: "Marcos is always a great professional to have around the club but he has not had a lot of chances to play this season because of the form and quality of Wayne Bridge.
"It's important for Marcos to be playing regular football so it was a good decision for him to move to Bournemouth on loan, where he can help them push for promotion out of League One. We hope that he has a successful remainder of the season."
Painter could make his debut for the League One leaders when they face Preston tomorrow.